Deck Joist Calculator
This free deck joist calculator is a focused takeoff — not a whole-deck wizard. If you already know your beam and ledger are set (or you're using the Deck Beam Span Calculator to size those separately) and just need the joist package, this gives you the size, spacing, and count from IRC Table R507.6, using the exact same formulas as the Deck Calculator so the numbers always agree.
It also covers two things a whole-deck calculator typically skips: joist cantilever and lateral restraint. The maximum overhang past the beam is read directly from Table R507.6's own cantilever column — by species, size, and spacing, not a flat rule of thumb — plus the code's footnote allowance for a cantilever up to the joist's nominal depth regardless of the table. For the beam end, IRC R507.6.2 requires lateral restraint by joist hangers, blocking, or a rim joist; this calculator prices out whichever method you pick, including the exact fastener count for a rim joist.
Everything is grounded in IRC Table R507.6 (span and cantilever), R507.6.1 (bearing), and R507.6.2 (lateral restraint). Sizing only — no pricing, no signup. For the full deck materials list, use the Deck Calculator; for the beam and post from this joist span, use the Deck Beam Span Calculator; for the footing, use the Deck Footing Calculator.
Deck Joist Calculator
A joist-only takeoff — size, count, cantilever, lateral restraint, and a stock-length cut list, straight from IRC Table R507.6, R507.6.1, and R507.6.2. Free, no signup.
Deck size & framing
For the whole deck's materials list (decking, beam, posts, footings), use the Deck Calculator instead — this tool is joist-only.
Spacing is set by your decking's ICC-ES rating — 16" o.c. covers nearly all wood and composite decking; 24" is only rated for Deckorators Surestone.
Cantilever & lateral restraint
The two things a whole-deck calculator usually skips.
IRC R507.6.2 requires lateral restraint at every joist bearing location — by joist hangers, blocking (≥60% of joist depth), or a rim joist nailed to each joist end. The ledger side is normally hangers (see the Deck Calculator); pick your method for the beam side here.
Joist span & cantilever table — 16" o.c.
Maximum span and maximum cantilever by joist size, per IRC Table R507.6 (No. 2 grade, wet service, 40 psf snow / live load). Footnote f also always permits a cantilever up to the joist's nominal depth regardless of this table. Change species or spacing above to see other columns.
| Joist size | Max span | Max cantilever |
|---|---|---|
| 2x6 | 9'-0" | 1'-4" |
| 2x8 | 11'-10" | 2'-3" |
| 2x10 | 14'-0" | 3'-6" |
| 2x12 | 16'-6" | 4'-2" |
Sizing the whole deck? Use the Deck Calculator. Sizing the beam and post from this joist span? Use the Deck Beam Span Calculator.
Span, spacing, and where the joist sits
Joists run from the ledger at the house to the beam, carrying the decking above and transferring load down through the beam, posts, and footings. This is where a joist fits in that chain — and what “span” actually means when you enter it above.
Span and spacing are two different measurements people mix up — spacing is the distance across the joists, span is how far each one reaches. This table shows how much farther a bigger joist, or tighter spacing, actually gets you.
The joist cantilever (how far a joist can overhang past the beam) is a completely separate limit from the beam cantilever (how far the beam can overhang past its post) — confusing the two is the most common cantilever mistake on a deck.
Calculation Formulas
The joist size is read directly from IRC Table R507.6, indexed by species group and spacing. At ground snow loads of 60 psf or higher, the size steps up one nominal size for safety margin (exact compliance needs the R507.6(2)/(3)/(4) sub-tables).
Example:
16-ft joist span at 16" o.c., Southern pine → 2×12 (16'-6" allowed).
On-center spacing creates one fewer bay than joist, so the count is the number of spacing intervals across the deck width, plus one for the outer end joist. A freestanding deck (two beam lines, no ledger) doubles every joist and every piece of restraint hardware.
Example:
12-ft wide deck at 16" o.c. → ceil(144 ÷ 16) + 1 = 10 joists.
The cantilever a joist may overhang past its beam is read from the table's own cantilever column (species/size/spacing specific — NOT a flat fraction of the span). Footnote f separately permits a cantilever up to the joist's nominal depth regardless of the table, as a baseline allowance.
Example:
2×10 at 16" o.c., Southern pine → 3'-6" maximum cantilever (Table R507.6).
R507.6.2 requires every joist bearing location to resist rotation, satisfied by any of three methods: joist hangers, blocking between joists at least 60% of the joist depth, or a rim joist nailed to each joist end with at least three 10d nails or three No. 10 × 3" screws.
Example:
A rim joist across a 12-ft-wide deck, nailed 3× per joist end → 30 nails for 10 joists.
IRC R507.6.1 sets the minimum bearing at each joist end. Joists on a multi-ply beam or ledger are fastened per Table R602.3(1); on a single-ply beam or ledger, by a mechanical connector; framing into the side of a beam or ledger uses approved joist hangers.
Example:
A joist resting on a drop beam needs at least 1.5" of the joist's width bearing on the wood.
Each joist is cut to the deck's joist span plus any cantilever, then rounded up to the nearest commonly stocked pressure-treated length (8/10/12/14/16/20 ft) for a purchase list.
Example:
13-ft span + 2-ft overhang = 15 ft required → buy 16-ft stock.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Species groups | Southern pine / DF-L,HF,SPF / Redwood,cedar | IRC Table R507.6 groups species into three columns for joist span AND cantilever — same grouping used by JOIST_SPAN_FT elsewhere on this site. |
| Snow step-up | +1 nominal size at ≥60 psf | A conservative safety margin for heavier snow; exact IRC compliance at 60+ psf uses the R507.6(2)/(3)/(4) sub-tables (engineered). |
| Cantilever floor (footnote f) | Up to nominal joist depth, always | A baseline cantilever allowance independent of the table — e.g. up to 10" for any 2×10, regardless of species or spacing. |
| Blocking minimum depth | 60% of joist depth | R507.6.2 — blocking used for lateral restraint must be at least this deep to be effective. |
| Rim joist fastening | 3× 10d nails or 3× #10×3" screws / joist end | R507.6.2's minimum fastening for a rim joist providing lateral restraint. |
| Bearing minimums | 1.5" wood/metal, 3" concrete/masonry | R507.6.1 — measured over the full width of the joist. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC Table R507.6 — Deck Joist Spans & Cantilevers(IRC R507.6 / Table R507.6)
View StandardThe prescriptive table sizing deck joists and their maximum cantilever, by species group, size, and spacing, for a 40 psf live / 10 psf dead load.
Key Requirements:
- •Joist size and cantilever read by species group, size, and spacing
- •No. 2 grade lumber, wet service factor
- •Footnote f: a cantilever up to the joist's nominal depth is always permitted
- •2021+ adds 50/60/70 psf snow sub-tables (R507.6(2)/(3)/(4))
IRC R507.6.1 — Deck Joist Bearing(IRC R507.6.1)
View StandardSets the minimum bearing at each joist end and how joists must be fastened depending on what they bear on.
Key Requirements:
- •1.5" minimum bearing on wood or metal, full joist width
- •3" minimum bearing on concrete or masonry
- •Multi-ply beam/ledger: fastened per Table R602.3(1)
- •Single-ply beam/ledger: mechanical connector; side-framing: approved joist hangers
IRC R507.6.2 — Deck Joist Lateral Restraint(IRC R507.6.2)
View StandardRequires every joist bearing location to be restrained against rotation, by one of three approved methods.
Key Requirements:
- •Joist hangers, OR
- •Blocking between joists at least 60% of the joist depth, OR
- •Rim joists nailed to each joist end with 3+ 10d nails or 3+ No.10×3" screws
- •Required at every bearing location, not just the beam
AWC DCA 6 — Prescriptive Residential Wood Deck Guide(AWC DCA 6)
View StandardThe American Wood Council's deck guide the IRC deck provisions are built from.
Key Requirements:
- •Scope: attached, single-level residential decks
- •Joist framing consistent with R507.6 span/cantilever tables
- •Detailing for hangers, blocking, and rim joists
- •Hot tubs and other concentrated loads are out of scope
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Snow Load
40 to 70+ psf
Heavier ground snow load shortens the allowable joist span and cantilever for the same size and spacing. This calculator applies a conservative one-size step-up at 60 psf and above; exact compliance at higher snow loads uses the IRC's dedicated snow sub-tables.
Regional Examples:
Species Availability
What's stocked locally
Southern pine dominates in the South and East, Douglas fir-larch/hem-fir/SPF in the West and Midwest, and redwood/western cedar on the West Coast. The table groups these into three spans and cantilevers — pick what's actually available at your lumberyard.
Regional Examples:
Restraint Method
What your inspector expects
All three lateral-restraint methods (hangers, blocking, rim joist) are code-equal, but local habit varies — some inspectors expect a continuous rim joist as the default, others accept blocking freely. Confirm which your AHJ prefers before framing.
Regional Examples:
Beyond the Prescriptive Table
When you need an engineer
Joist spans or cantilevers beyond the largest tabulated values, joists continuous over more than two supports, and any concentrated load (hot tub, built-in seating) fall outside this table and need an engineered design.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
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Related Calculators
Deck Calculator
Code-compliant deck framing, decking, footings, ledger, stairs & railing — built on 2021 IRC R507 and AWC DCA 6 tables. Free, no signup.
Deck Beam Span Calculator
How far can a deck beam span? Free calculator for beam size, ply, post spacing, footing & cantilever from the joist span it carries — per IRC R507.5 / DCA 6.
Joist Span Calculator
Free joist span calculator: max spans for floor and deck joists by species, grade, size, and spacing per IRC R502.3.1 and R507.6. No signup.
How to Use This Calculator
- Enter your deck length (ledger to beam — the joist span) and width.
- Pick your wood species and joist spacing (set by your decking's rating — 16" o.c. covers nearly everything).
- Set your ground snow load and the joist stock length you want to buy.
- If any joists overhang the beam, enter the overhang — the calculator checks it against the code's cantilever limit.
- Choose how you'll provide lateral restraint at the beam end: a rim joist or blocking between joists.
- Click Calculate for the joist size, count, cantilever check, restraint bill of materials, and cut list.
Cantilever and Lateral Restraint — the Two Things People Miss
Most deck framing guides stop at "here's your joist size and count," but the IRC has two more rules that trip people up. First, cantilever: a joist can overhang past its beam, but not by an arbitrary amount — IRC Table R507.6 gives an exact maximum cantilever for each species, size, and spacing combination, and it is NOT simply one-quarter of the span (that rule applies to beams, not joists — a completely separate limit). A 2×10 Southern pine joist at 16 inches on-center can cantilever 3 feet 6 inches, while the same size at 24 inches on-center drops to 2 feet 10 inches, because wider spacing puts more load on each joist. There is also a fallback: any joist can cantilever up to its own nominal depth (10 inches for a 2×10) even if the table would otherwise be more restrictive. Second, lateral restraint: every joist end needs something keeping it from rolling over sideways under load, and the code gives you three equally valid options — a joist hanger, blocking between the joists at least 60 percent as deep as the joist, or a rim joist nailed to every joist end with at least three 10d nails or three number-10 screws. None of these is "more correct" than the others; pick based on what you have on hand and what your inspector expects to see.
Frequently Asked Questions
How many joists do I need for my deck?
Count = ceil((deck width in inches) ÷ spacing) + 1 — the number of on-center spacing intervals across the deck, plus one for the outer end joist. A 12-foot-wide deck at 16 inches on-center needs ceil(144 ÷ 16) + 1 = 10 joists. A freestanding deck (two beam lines, no ledger) doubles that. Enter your deck width, species, and spacing and this calculator gives you the exact count, using the same formula as the Deck Calculator.
How far can a deck joist cantilever past the beam?
It depends on the species, size, and spacing — IRC Table R507.6 gives an exact maximum cantilever for each combination, not a flat fraction of the span. A Southern pine 2×10 at 16 inches on-center can cantilever 3 feet 6 inches; the same joist at 24 inches on-center drops to 2 feet 10 inches. This is a completely separate rule from the beam cantilever (which IS one-quarter of the beam's span) — don't confuse the two. The code also has a fallback: any joist can cantilever up to its own nominal depth (10 inches for a 2×10) even where the table is more restrictive.
Does blocking or a rim joist matter more for a deck?
Neither — IRC R507.6.2 treats joist hangers, blocking, and a rim joist as three equally valid ways to provide lateral restraint at a joist's bearing location, so long as you meet each method's spec: blocking must be at least 60 percent of the joist's depth, and a rim joist must be nailed to every joist end with at least three 10d nails or three number-10 by 3-inch screws. Pick whichever fits your framing and what your inspector expects to see — this calculator prices out either choice.
What size joist do I need for my deck?
Joist size comes straight from IRC Table R507.6, indexed by species, spacing, and the span (ledger to beam). For pressure-treated Southern Pine at 16 inches on-center: a 2×6 spans 9 feet, a 2×8 spans 11'-10", a 2×10 spans 14'-0", and a 2×12 spans 16'-6". Douglas Fir-Larch/Hem-Fir/SPF and Redwood/Western Cedar span somewhat less. At ground snow loads of 60 psf or higher, step up one nominal size for a safety margin — exact compliance at that point uses the code's dedicated snow sub-tables.
How deep does joist bearing need to be?
At least 1.5 inches on wood or metal, and at least 3 inches on concrete or masonry, over the full width of the joist (IRC R507.6.1). How the joist attaches depends on what it bears on: a multi-ply beam or ledger uses the fastening schedule in Table R602.3(1); a single-ply beam or ledger needs a mechanical connector; and a joist framing into the SIDE of a beam or ledger (rather than resting on top) always needs an approved joist hanger.
What's the difference between this and the Deck Calculator?
This calculator is a joist-only takeoff — size, count, cantilever, lateral restraint, and a stock-length cut list, with the code details spelled out. It's built for re-joisting an existing deck or a small addition where the beam and ledger are already settled. The full Deck Calculator produces a complete materials list for a new deck: decking, joists, beam, posts, footings, ledger fasteners, stairs, and railing. Both use the identical IRC Table R507.6 joist formulas, so the joist numbers always match between them — use this one when you only need the joist package.
Can I use this calculator instead of the Deck Beam Span Calculator?
No — they answer different questions. This calculator sizes the JOISTS that span from the ledger to the beam. The Deck Beam Span Calculator sizes the BEAM itself (and its posts and footing) from the joist span this calculator gives you. Use this one first to confirm your joist span and size, then plug that span into the Deck Beam Span Calculator to size the beam that carries them.